Conditionally select compiler based on its version - c++

Question

Goals
Make cmake select clang++ if its version is above 10; otherwise, use g++. If clang++ is below 10 and the default g++ is below 10.1, use g++-10 (REQUIRED).
This is to achieve compiler fallback.
Background
As kindly noted in this answer, one should set the default compiler before the project keyword.
Instead of using
-DCMAKE_CXX_COMPILER=clang++
or
set(CMAKE_CXX_COMPILER clang++)
I used the following statements with reference to the answer here:
find_program(CMAKE_CXX_COMPILER
NAMES $ENV{CXX} clang++ PATHS ENV PATH NO_DEFAULT_PATH)
if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 10)
find_program(CMAKE_CXX_COMPILER
NAMES $ENV{CXX} g++ PATHS ENV PATH NO_DEFAULT_PATH)
if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 10.1)
find_program(CMAKE_CXX_COMPILER
NAMES $ENV{g++-10} g++-10 PATHS ENV PATH NO_DEFAULT_PATH REQUIRED)
endif()
endif()
Problem
The compiler is not set to g++ when the version of clang++ is less than 10, but I could check that cmake passes the branch
if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 10)
I do not understand this behavior. And are there any alternative to this without using command line arguments?
UPDATE
This question was asked in acknowledgment of the fact that setting a compiler in CMakeLists.txt is discouraged.
#KamilCuk suggested two ways to achieve what I stated, and one may take those suggestions if they want to.
Here's what I did.
TL;DR
Do not specify a compiler in CMakeLists.txt.
My suggestion
This is contrary to what I have previously done in this question.
I scrapped all the configuration logic from CMakeLists.txt and decided to create a shell file that extracts compiler versions and select an appropriate compiler. This way of configuration is quite well-known, which can be found in many popular repositories such as ImageMagick and tensorflow.

Answer 1

If you want to write the logic in cmake, you could write something along:
cmake_minimum_required(VERSION 3.11)
# Before project()!!!
execute_process(COMMAND clang++ --version OUTPUT_VARIABLE result)
string(REGEX REPLACE "[^0-9]* ([0-9]+).*" "\\1" clang_version "${result}") # extract the version with some regex
execute_process(COMMAND g++ --version OUTPUT_VARIABLE result)
string(REGEX REPLACE "[^0-9]* ([0-9]+).*" "\\1" gcc_version "${result}") # extract the version with some regex
# write your super condition here and set CMAKE_CXX_COMPILER depending on it
set(CMAKE_CXX_COMPILER g++)
if (gcc_version LESS 10)
set(CMAKE_CXX_COMPILER clang++)
endif()
project(blabla) # uses CMAKE_CXX_COMPILER and configures cmake
You could also use include(CMakeDetermineCXXCompiler) to determine the version, or use the template files from cmake from CMakeCXXCompilerId.cpp.in from cmake installation path that is used by CMakeDetermineCXXCompiler internally.